It starts in medical school, regularly appears in your medical training, sneaks around nursing schools and is an impetus for discussions in the ICU: The great myths about Hemoglobin (Hb) and Hematocrit (Hct).

These two haematological lab-parameters are part of our daily life at work and are mostly measured together... as a package. Some clinicians look at haemoglobin levels, others prefer hematocrit levels... but then there is always someone making a great deal of differentiating between the two parameters and making all sort of diagnostic conclusions. 'Hct is better to determine dilution of the patient' or 'Acute blood loss is better determined by Hb than Hct'... and so on.

So here's the question: What actually is the difference between Hb and Hct? Do we need to measure both in clinical practice?

What's the difference?

Hemoglobin levels are mostly measured by automated machines designed to perform different tests in blood. Within the machine, the red blood cells are broken down to get the haemoglobin into a solution. The concentration of haemoglobin is then measured by spectrophotometry using the methemoglobin cyanide method.

Hematocrit levels in contrast are actually calculated by an automated analyzer... It is actually not measured directly! The analyser multiplies the red blood cell count by their mean corpuscular volume.

What is Fact?

There simply is NO difference between Hemoglobin and Hematocrit by means of clinical information!

In fact, virtually all haemoglobin in our blood is contained within erythrocytes

Therefore, whether the amount of Hb per litre of blood is determined or the blood’s volume occupied by the Hb filled erythrocytes is determined, similar information is gained.

Nijboer at al. have brilliantly proven that Hb and Hct correlate in all ranges and all patients and also nicely show this in their figure 1 (see below)

The only rare exceptions are macrocytic and polycytemic anaemia in which the Hct is defined by erythrocytescontaining a normal mean corpuscular Hb concentration

Idiopathic pulmonary fibrosis is one of these frustrating diseases you repeatedly encounter in the ICU and that mostly leaves you sort of frustrated at the end. Despite all the efforts in research we are still left with very little we can do. This is one reason why also intensivists need to keep themselves updated on this topic.

As knowledge is growing the ATS, ERS, JRS and ALAT (... thoracic and respiratory societies) made the effort to look into the latest evidence by performing systematic reviews and where appropriate meta-analyses. The aim was to update the guidelines published in 2011. These guidelines are also dedicated to Mr. William Cunningham who actively participated in the development of these guidelines, suffered from idiopathic pulmonary fibrosis for many years and who was directly confronted with the issues related with this condition.

When performing a kidney transplantation nowadays up to 50% of recipients developed a delayed graft function which is defined as the need of dialysis within seven days. The authors of this recently published NEJM-article asked themselves whether mild hypothermia might influence outcome in this regard.

In order to answer this question the investigators assigned organ donors after declaration of death according to neurologic criteria into two groups. They were either treated with mild hypothermia (34 to 35°C) or with normothermia (36.5 to 37.5°C). The target temperature was maintained until the patients were transferred to theatre for transplantation.

Primary outcome of this trial was delayed graft function among recipients. Secondary outcomes included the rates of individual organs transplanted into each treatment group at the total number of organs transplanted from each donor.

This trial had to be stopped early as an interim analysis showed significant efficacy of mild hypothermia. Up to this point a total of 572 patients received a kidney transplant (285 in the hypothermia group and 287 in the normothermia group). 28% of recipients in the hypothemia group developed delayed graft function compared to 39% in the normothermia group.

This study suggests that potential organ donors after declaration of death according to neurologic criteria should be treated with mild hypothermia.

Intensive care units that continue to treat patients with mild hypothermia after cardiac arrest might have two rewarm their patients for the diagnosis of neurological death before re-cooling them for organ transplantation

Beverley Hunt at al. have just published an excellent practical guideline for the haematological management of major haemorrhage which also serves a a great educational review on this topic... an excellent piece of work!

The authors look at this topic point for point and review current literature in an easy to understand sort of manor. They define major blood loss when it leads to a heart rte of >110/Min or a systolic blood pressure of less than 90mmHg, or simply said: when bleeding becomes haemodynamic relevant. In general it is recommended to have a major haemorrhage protocol at hand (1D) and all staff should be trained to recognise major blood loss early (1D).

Here's a summary of the recommendations made by the British Committee for Standards in Haematology (BCSH):

In Major Haemorrhage....

Red Blood Cells RBC- Hospitals must be prepared to provide emergency Group 0 red cells and group specific red cells (1C)- Patients must have correctly labelled samples taken before administration of emergency Group 0 blood (1C)- There is NO indication to request 'fresh' or 'young' red cells (under 7d of storage, 2B)- Note: The optimum target haemoglobin concentration (Hb) in this clinical setting in general is NOT established. Current literature shows a tendency towards restriction towards 70-90g/L, but the BCSH makes no recommendations therefore (see blow)

Haemostatic Monitoring- Use haemostatic tests regularly during haemorrhage, every 30-60min, depending on severity of blood loss (1C)- Measure platelet count, PT, aPTT (1C)- Note: The BCSG does not recommend TEG and ROTEM at this stage

Fresh Frozen Plasma FFP- Use FFP in a 1:2 ratio with RBC initially (2C)- Once bleeding is under control administer FFP when PT and/or aPTT is >1.5 times normal (recommended dose 15-20ml/kg, 2C)- The use of FFP should not delay fibrinogen supplementation if necessary (2C)

Tranexamic Acid TA- Give tranexamic acid as soon as possible to patients with, or at risk of major haemorrhage (Recommended dose: 1g IV over 10min, followed by 1g IV over 8h, 1A)- Note: TA has no known adverse effects- Note: Aprotinin is not recommended

Recombinant Activated Factor VIIa (Novo Seven)- Do not use

Specific Clinical Situations

Obstetrics- Fibrinogen levels increase during pregnancy to 4-6g/L- In major obstetric haemorrhage fibrinogen should be given when levels are <2.0g/L (1B)

GI-Bleed- Use restrictive strategy for RBC transfusion is recommended in most patients (1A)

Trauma- Transfuse adult trauma patients empirically with a 1:1 ratio of FFP : RBC (1B)- Consider early use of platelets (1B)- Give tranexamic acid as soon as possible (Dose 1g over 10min and then 1g over 8h, 1A)

Prevention of Bleeding in High-Risk Surgery- Use tranexamic acid (Dose 1g over 10min and then 1g over 8h, 1B)